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 Repairing and Servicing Lennox Products
 Repairing / Servicing Lennox Gas Furnaces
 G40DF Inducer Blower Problem
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SEAN

2 Posts

Posted - 01/21/2008 :  9:13:36 PM  Show Profile  Reply with Quote
My Lennox G40 will not ignight:
1. In the start up sequence the inducer moter energizes briefly but, not for the second or so that I understand it should. I mean it may move 5 degrees clockwise.
2. The pressure switch is open when checked with an ohm meter. I've removed the switch and by blowing into the switch I can activate and de-activate the switch. Verified with an ohm meter.
3. I've inspected the tube between the induction moter and the switch. All is fine.
4. with the circuit breaker disconnected I removed the wires from the induction motor and applied 110VAC and the motor runs. I believe this verifies the motor. At the same time I verified that there is air flow out of the connection to the pressure switch. I hooked the switch up and it had no trouble activating it in my "make shift" test mode.
5. The LEDs are 1=off and 2 flashes slowly.

During startup mode should the inducer moter spin up for a second fast enough to activate the pressure switch? Does this set some kind of timing up on the circuit board? I noticed the induction moter is 3000RPMs. It took a good 3 to 5 seconds in my "make shift" test mode before it would have enough pressure to activate the switch.

so here's the bottom line. The intake and exaust is clear. The moter for all intents does not activate on start up. So let me know what you think. I don't mind spending the money on the parts and was an ace radar electronics guy in the military. I just hate to buy the wrong part. I'm thinking I need to replace the circuit board. The solid state relay (K3) on the 56L8401 board is probably bad but I don't have a true schematic for the board to see what actually energizes K3. Any input would be held in the highest regard.

Cozy.Support

5773 Posts

Posted - 01/21/2008 :  9:55:52 PM  Show Profile  Reply with Quote
On a call for heat, the induced draft blower should start and run. If the induced draft motor does not run, it will set a pressure switch diagnostic code. It will run quite a while during the try for ignition.

Lennox does not distribute board level schematics. Some boards have a relay that turns the induced draft blower on while others use a triac.



Cozy.Support does not provide a diagnosis for users of this forum. We do, however, try to provide information that will help servicers arrive at their own diagnosis.

Once you know what part you need, you can contact the Cozy parts department by using the link below
http://www.cozyparts.com/Lennox-Parts/locator_form.aspx

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slctech

2085 Posts

Posted - 01/22/2008 :  01:16:48 AM  Show Profile  Reply with Quote
Being an integrated control board, a majority of the functions are controlled and monitored by the pre-programming on the board's microchips. K3 relay is designed to be energized immediatly upon 24v being applied to the terminal "W" on the thermostat strip. The relay is to stay energized as long as a constant 24v is applied. This would be from the beginning of the heat stage to the end of the heat stage when the 24v demand is removed from the "W" terminal.
It appears that you have done a satisfactory job identifying the issue. I would double check by jumpering R & W and with a your meter attached to the inducer leads, you should immediatly see 110-120v to the inducer. If not it is apparent you have a bad board.
There is no need to troubleshoot the board any further.

FYI when checking the voltage from the board to the motor, do it directly from the harness at the board. There may be issues somewhere in the wiring or harness connection points at the molex plug between the board and the motor.

As a Lennox Technician I feel basic repairs and maintenance can be performed by the homeowner. But I highly suggest that periodic maintenance should be made by a certified licensed technician. You should contact your local Lennox technician for any repairs or issues that are outside your comfort zone. Property damage, personal injury, and death can occur from improperly repaired or maintained systems. Replacing a part doesn't always resolve the issue that caused the part to fail!

Edited by - slctech on 01/22/2008 01:19:33 AM
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SEAN

2 Posts

Posted - 01/22/2008 :  3:36:53 PM  Show Profile  Reply with Quote
After swapping boards and having the same issue. I started over. It turned out to be a primary limit switch. When I originally checked it with my ohm meter it read continuity (or so I think). I thought I would follow the circuit with power applied.

1. I put my meter on DC. Put my neg lead on ground and then walked down the line of switches...left rollout...one side 24v-other side 24v...right rollout...one side 24v-other side 24v...primary limit...one side 24v other side 0v.
2. I removed power and jumped the switch just to test the solution...it worked.
3. I replaced the primary limit switch and all is warm.
What I learned was that any break in that line of switches, which will pervent the inducer motor from starting will give you the LED fault indication described in my original message. The book say a possible pressure switch fault but, it can also be one of the roll out switch or a primary limit switch. Anything in that line for that matter.
A QUICK SAFTEY FOOTNOTE: NEVER LEAVE A SWITCH LIKE THAT JUMPERED OUT OF THE CIRCUIT! The outcome could be catastrophic!
Thank you for your assistance in helping me think through this problem.
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